Patients who received a diagnosis of IRF will be enrolled, while patients with secondary forms of retroperitoneal fibrosis (e.g. drugs, infections, radiotherapy) will be excluded. When present, ureteral obstruction will be managed by ureteral stents/nephrostomy/ureterolysis. All patients will then receive oral prednisone (1 mg/kg/day) for one month, at the end of which they will be randomized to receive either tamoxifen (0.5 mg/kg/day at fixed dose for 8 months) or prednisone (0.5 mg/kg/day for the first month, 0.25 mg/kg/day for the second and third months, and then tapered off during the ensuing 5 months). A CT/MRI study will be performed before the start of treatment, four months after randomization and at the end of treatment. All patients will be followed up for at least 18 months after the end of treatment.